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Carbon dots (CDs) have attracted increasing interest in recent years owing to their desirable properties. Despite the availability of diverse elaborate CDs, the function and application of CDs are far to be fully exploited. Here, biomass-derived carbon dots dispersed in a polymer matrix are found to behave as ink-free patterned substrates, which are demonstrated to be useful for nondestructive collection and recognition of latent fingerprints (LFPs), as well as printing. The coating of CD/poly(vinyl alcohol) solution on a LFP yields a flexible transparent film; a stable fluorescent fingerprint with clear ridge details enabling personal identification is formed on this film. Encouragingly, this method can be applied to nondestructively lift and recognize long-timely exposed LFPs from various surfaces. The mechanism for LFP collection and visualization is proposed, which should be ascribed to the interfacial segregation of CDs in the polymer matrix during the film forming process. This mechanism is further validated by and utilized for application of CD/polymer composites in relief printing, intaglio printing, and micro-trace transferring.

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An effective method for preparing two dimensional responsive photonic crystals (RPCs) is presented in this work. Specifically, the negatively-charged latex colloids crystallize out quickly from the highly concentrated colloid dispersions as they come into contact with the similarly charged hydrophilic substrate, and two dimensional close-packed RPC assemblies with bright structural colors are then attained on the substrate surface. Combining with the poly(acrylamide-co-acrylic acid) hydrogels, an ultrathin RPC sensor along with an excellent appearance and optical evenness is finally obtained. When applied to trace electrolyte sensing, it shows highly precise and quick response characteristics. These features may make the RPCs an ideal candidate of a chemical classifier for naked-eye water quality analysis. Besides, by using a stepwise polymerization method, the resulting RPC film can be further used to convey barcode messages. The encoding messages vanish at the dried state but emerge when the film surface is smeared with water, implying potential applications in anti-forgery areas.